Most human cells contain 46 chromosomes, or 23 pairs of homologous chromosomes. Each chromosome of the pair is the same shape and size and has the same genes in the same location. Chromosome disorders occur due to two types of alteration in the chromosomes. Chromosome disorders can be caused by an alteration in the number of chromosomes in the nucleus, or by an alteration in the structure of a chromosome.

Sex cells, or gametes, have only 23 chromosomes. When the sex cells are produced, chromosomes separate and move to opposite ends of the cell. Sometimes a chromosome moves to the same end of the cell as its pair, instead of the opposite end. This extra chromosome is incorporated into the nucleus of the daughter cell, causing it to have an extra chromosome, a nondisjunction. As a result, one of the gamete cells will have two copies of the chromosome and the other will have none.

During fertilization, a sperm cell merges with an egg cell producing a zygote with two copies of each chromosome. If a nondisjunction has occurred, and a gamete with the wrong number of chromosomes unites with another, the resulting cell will have the wrong number of chromosomes. If the cell has too many chromosomes, it is said to be polyploidy. Often, the cell only has one extra copy of a chromosome, or three in humans, so it is called a trisomic cell. If the cell only has one copy of a chromosome, it is called a monosomic cell.

Nondisjunctions in human cells are relatively high. The results are often lethal to the developing fetus though, so it usually doesn't survive. Chromosome disorders caused by nondisjunctions that do result in children being born are:

• Down syndrome – An extra chromosome 21 is present so it is also called trisomy 21. The affects range from moderate to severe and those with Down syndrome have characteristic facial features, a short stature and heart defects. They often suffer from respiratory diseases, have a shorter life span and some degree of mental retardation.
• Patau syndrome – This syndrome results from a trisomy of chromosome 13. It causes severe eye, brain and circulatory defects. Cleft palate is often a result and the children rarely live longer than a few months.
• Edward’s syndrome – Children with Edward’s syndrome rarely live longer than a few months, as all of their organs are affected in some way. This condition is caused by trisomy 18.
• Klinefelter’s syndrome – Individuals with this syndrome have multiple copies of the X chromosome, XXY, XXXY or XXXXY. These individuals are male, but the presence of extra X chromosomes causes body proportions that are female and smaller testes with no sperm development. The greater the number of X chromosomes, the more marked the condition.
• Turner’s syndrome – Children with Turner’s syndrome have only one X chromosome, so they have only 45 chromosomes in total. This is the only non-lethal monosomy in humans, but most do not survive the pregnancy. Those that are born are female, but they are small in stature and do not mature sexually.

Changes in structure are what cause the other chromosome disorders. There are four changes that can occur in chromosome structure to cause chromosome disorders – deletions, inversions, translocations and duplications. These structural changes are due to chromosomes breaking and then not reattaching correctly.

During a deletion, a part of the chromosome is lost. As a result, this can cause a loss of the genes on that portion of the chromosome. Losing genes can significantly affect an organism’s development and can often prove lethal. Cri du chat is a chromosome disorder caused by the deletion of part of chromosome 5. It results in severe mental retardation, a very small head with unusual features and the child makes a distinct cry that sounds like an upset cat.

During an inversion, a portion of the chromosome is reattached in the inverted position, causing the sequence of the genes on this portion to be reversed. While the overall genetic information is the same, the characteristics affected by the genes may be changed. If a portion of a chromosome is reattached at a different point on the same chromosome, or to a new chromosome, a translocation has occurred. Like an inversion, this can also cause changes in the characteristics of the individual depending on how, where and what was moved.

A duplication occurs when a portion of the chromosome is doubled, resulting in a repetition of the gene sequence. The most common form of mental retardation in humans is caused by a duplication at the tip of the X chromosome. People with Fragile X can have more than 700 repeats instead of the usual 29 seen in normal X chromosomes.